So for 3 seconds of an ability with a giveaway sound effect (even if you can't see the particle), you get 105 base damage, .3 AP, and .25 seconds of knock-up. The base damage is abysmal when uncharged and only average when fully charged. The CC's main benefit is reliability, but that's when it's fired off instantly. There's absolutely no incentive to try and charge this ability.

Since Riot seems to want players to charge Janna's Q and Ultimate when it becomes optimal to do so, I think I'd rather see some meaningful buffs to Q as opposed to overpowering her other abilities and leaving her with a half-ass'd Q.

Suggestions (not all at once, just things that can make it better):

• Let the cooldown of the ability start ticking down ala Lux's E/Gragas's Q. This does not require compensating by increasing Q's cooldown as it is already comparable in CD to similar options. As it is, charging Janna's Q may as well be read as "increase the cooldown of this ability by 3 seconds to miss it".
• Knock-up scales from .75 to 1 second from no charge to 2.99s charged. At 3s charged, the ability knocks up for 1.5 seconds instead. Additionally, the ability deals an extra 70 (+.2 AP) magic damage for a total of 335 + .85 magic damage. This is similar to how Nunu's ultimate works. if this will destroy minion waves too fast, compensate with ~25-50% reduced damage to minions.
• Decrease the charge time from 3 seconds to 2 seconds. Alter damage ramp to compensate.
• Casting Howling Gale within 5 seconds after casting Monsoon gives you a fully charged Howling Gale that fires immediately. Probably a bit nonsensical, but at least it gives her a chance to hit enemies with a fully charged Q.
• Have the missile speed ramp up with charge, from what it is now at no charge to about twice as fast at full charge.
• Lower the mana cost from 90/105/120/135/150 to 80/90/100/110/120. Because let's be honest, this ability is horribly overpriced.
• EDIT: Have the tornado display the path it is going to travel while charging. Outside of being a clarification buff, this also gives your allies an opportunity to try and set up for the stronger charge.

I could probably go on with other potential buffs, but I think it speaks for itself. There's no incentive to charge Janna's Howling Gale. There's almost no situation where charging it for minor damage and CC buffs is better. If it weren't for the fact that ranking up the ability decreases the cooldown, I'd leave it at rank 1 just to save myself 60 mana per cast.